The invention relates to a ground circuit in a low-energy system. The invention also relates to a method for recovering energy.
A ground circuit and a method of this kind are utilized, in particular, in systems where energy—as well heat as cold—is transferred with a terminal device from ground, rock or water by the intermediary of a transfer fluid. Said terminal device may be as well a heat pump as an air conditioning radiator.
In this context the low-energy system refers to a system whose energy source has a low temperature, and most conventionally this temperature may be in the range of +2 to +10° C. In this specification, the energy content generated by an energy source, such as ground, rock or water, will be referred to as low energy. Utilization of the low energy of ground, rock or water has generally referred to heating of a building or tap water by employing a heat pump or various heat collection circuits, for instance. There is conventionally obtained 2 to 4 units of heat per each electric energy unit used. In cold climate conditions the heating energy consumption for buildings is considerable, whereby the utilization of low-energy systems is becoming more and more economically viable as the costs of electricity and oil increase.
Naturally, the present collection circuit and collection method may also be utilized in cooling indoor spaces. In that case a cool transfer fluid coming from a heat collection circuit is circulated, for instance, through cooling beams, cooling radiators or the like devices.
Up to the present, a commonly used manner to recover energy has been to place a collection circuit, i.e. so-called ground circuit, in a soil layer surrounding a building, where it is buried in a frost-free depth substantially horizontally. A ground circuit of this kind requires a large surface area to obtain a sufficient efficiency, and consequently it can only be used on large plots of land. Pipe loops in a ground circuit shall be at least 1.5 m apart from one another so that the adjacent loops would not interfere with the energy recovery of one another. To place a horizontal pipe system in the ground requires that an extensive pipe trench system be dug throughout the length of the ground circuit, whereby the placement thereof in a finished courtyard area or park, for instance, is difficult without causing serious damage to the root systems of plants and trees.
A second manner to recover energy is to place a collection circuit in the bottom of a lake or another water body, whereby energy is transferred from the bottom sediment and water to a transfer fluid. The collection circuit may be conveyed into water on land, but in that case an outgoing pipeline and a return pipeline should have separate, specific trenches. The collection pipe system placed in water is easy to install in the bottom of the water body. However, a liquid-filled pipe is lighter than the surrounding water, and consequently it tends to rise towards the surface. Irregularly risen portions of the collection pipe system may produce in the collection pipe system air pockets that hamper the circulation of the transfer fluid. In order to provide steady energy yield the collection pipe system should be anchored to the bottom of the water body. The pipe system installed in the bottom of the water body is also more vulnerable to breakage than the one dug in the ground. For instance, an anchor of a boat or a similar device may get caught in the pipe system and break it. In the shore line the outgoing and incoming pipelines shall be buried sufficiently deep, so that the ice would not damage the pipe system in winter.
A third manner to recover energy, which is currently becoming more and more common, is to construct a so-called heat well. In that solution a special pipe system, which constitutes a ground circuit, is buried in a deep, vertical hole drilled preferably in rock. The heat well requires a very small surface area compared with a horizontal pipe system, and the amount of energy obtained therefrom is conventionally double compared with a collection circuit placed in a soil layer or in a water body. Energy yield is particularly good when the heat well is drilled in rock. It is common, however, that on top of the rock there is a significant layer of loose material, such as soil and/or rubble. This portion containing loose material increases the cost of a heat well, because it must be furnished with a special protective pipe which prevents the well from collapsing. In addition, energy yield from the loose material portion is poorer than from the rock portion, and consequently the well is to be made deeper, or there is to be made a plurality of wells side by side in the loose material.
Selection between these three collection methods depends on the location, surface area and soil of the available area. Because the construction of a collection circuit is labour intensive, the costs incurred have often been high, which for its part has curbed the interest in low-energy systems.